Use of long-term opportunistic surveys to estimate trends in abundance of hibernating Townsend's big-eared bats

The advent of broad-scale threats to bats such as white-nose syndrome and climate change highlights the need for reliable baseline assessment of their populations. However few long-term, rigorously-designed assessments of bats populations exist, particularly in western North America. Consequently, results of informal monitoring efforts are often the only data available upon which to base population assessments. We evaluated whether an opportunistic collection of surveys recorded over a 22-year period could be used to assess population trend of Townsend's big-eared bats (Corynorhinus townsendii) at Lava Beds National Monument in northern California. We used records of counts of hibernating bats conducted during 1991-2012 to estimate the number of bats in 52 individual caves as well as cumulatively. Seventeen of 22 caves surveyed in {greater than or equal to} 4 years had an increasing trend in the number of hibernating bats. We estimated the cumulative annual growth rate over the period t...